Process of making regenerated cellulose films



July 20, 1948. J. BI'NICHOLS 2,445,333

PROCESS OF MAKING REGENERATED CELLULOSE FILMS I Filed Nov. 25, 1944 i'fl a gg FQ 8 58-. g w Q INVENTOR. James B M31012 JV 1291;

ATTORNEY i atentecl July 20,

PROCESS OF MAKING REGENERATED CELLULOSE FILMS James Burton Nichols, Wilmington, Del., assignor to E. I. du Pont de Ncmours & Company, Wilmington, Del., a corporation of Delaware Application November 23, 1944, Serial No. 564,893

This invention relates to a process of making regenerated cellulose films.

Regenerated cellulose film, as obtained by the well known wet regeneration process, has achieved outstanding commercial success. Nevertheless, it is not free of certain shortcomings, of which the most serious are relatively poor tear resistance and unsatisfactory durability under conditions of low temperature and low humidity. The latter defect may be at least partially remedied by addition of softening agents to the film but this introduces another problem since moistureproof coatings do not adhere 'well to the softened film, particularly if it contains a high proportion of softening agent. 'Another disadvantage of regenerated cellulose film produced by the wet regeneration process resides in its non-uniformity in all three dimensions, the film having less elongation and lower tear resistance in the direction in which tension was applied during formation. The trade has long recognized the need for a regenerated cellulose sheet characterized by high tear resistance, uniformity in all directions, toughness under cold, dry conditions even without a softener, and good' anchorage for moistureproof coatings.

Therefore, an object of this invention is to provide a process for making regenerated cellulose films characterized by three-dimensional, random X-ray patterns, substantially uniform tear resistance in all directions and unusual toughness even under severe atmospheric conditions. Other objects will appear hereinafter.

These objects are accomplished, in accordance with this invention, by extruding or casting viscose on a support, evaporating water from the extruded viscose at a rate sufficient to reduce the Water content to less than 45% by weight, and preferably to less than before irreversible gelation takes place, i. e., before the film loses its water solubility, heating the resulting film in 6 Claims. (01. 18-57) a chemically inert atmosphere, e. g., air, at least until irreversible gelation is obtained, i. e., until the filrn is no longer soluble in water, stopping the heating when regeneration is substantially complete, washing out the soluble salts and drying the film.

The cellulose from which the viscose is pre pared is preferably any of the commercially available cotton linters or Wood pulps having good resistance to oxidative degradation and sufficiently high degrees of polymerization to be able to retain a degree of polymerization of at least 500 and preferably 550 to 600 glucose units in the viscose. Viscose containing cellulose having a degree of polymerization below 500 (which is normal for making commercial wet-cast cellulose sheets) may be used but the dry-cast films prepared therefrom have a substantially lower durability than films prepared from viscose made from unaged alkalki cellulose having a degree of polymerization of approximately 600. The salt index of the viscose should be above 1.0 and preferably above 2.0. Above this limit the salt index is not critical; however, high salt index viscose yields stronger but hazier films. When viscose having a salt index below 1.0 is used, coagulation occurs and regeneration is completed too early in the drying stage. The use of a small amount of wetting agent in the viscose is desirable as this tends to avoid the formation of pinholes in the film when the viscose is cast on a support.

The operating conditions necessary to obtain regenerated cellulose films of optimum toughness and durability must be carefully controlled. As illustrated hereinafter by the examples, the viscose is extruded onto any suitable support or casting surface at ordinary or elevated temperatures: It is essential, however, that the temperature of the casting surface be held below 120 C. and preferably below C. to avoid pro-- mature decomposition of the cellulose xanthate.

Furthermore, it is essential to rapidly reduce the water content of the film to less than 45 and preferably below 20% before suificient xanthate has been decomposed to produce irreversible coagulation of the cellulose. This point is at approximately 0.15 mol of combined xanthate per glucose unit of the cellulose as measured by iodine titration of the coagulated film after washing out sulfur compounds with cold sodium chloride solution. This can be accomplished only under certain carefully controlled conditions. The preferred expedient for accomplishing this rapid reduction in water content is to force a rapid current of air or other inert gas over the surface of the thin 1W6 of viscose. The rate of circulation or" the inert gas over the film must 'be more than 400ftJ/min. (relative'to the movement of the film), and to obtain optimum film properties it should be circulated at a rate or more than 1000 ft./min. When the speed'of the strength and unsatisfactory clarit The time required to dry,'i. elf'to reduee the initial water content of the viscose' films to the desired degree depends on the temperatureof the casting support and the temperature, humidity and rate of circulation of the inert gas. With both the support and the circulating gas at ordinary r'oorfitemperatur'e and a relative humidity ofapproximately 50 and an air speed of 1000 'ft/ininfa drying time of 30 minutes is required toreduce the water content ofthe iscose film to 10 -2070. In a' hot drum casting process 1-3 minutes are required when the drum temperature is"0 '-l10 C.,"the'circulating air temperature is roo -130 0., and the rate of circulation i 1000 ijtf/min. With intermediate temperatures and air speeds. the timeof'drying will be between the values just mentioned. "*"Aftei"the yiscose film is dried to the degree specified above, the cellulose xanthate film is sir ecteii to further heating to substauoitiall'y dees pose it, i.'e;, to regenerate the cellulose. This can be'done'in combination with the drying stage by iurtlierheatingof the dried film withbutits f'moyal' from the support. However; for pract i'cal'ifeasons it ispreferred tostrip the dried film fr" the casting supportas'so'o'nhs the cellulose so tentl'ias reached 30-40% '(which'is equiv an; 't' to"a' reduction in 'moistu're corite'ntto 3Q 10%). The stripped filrn "is' then subjected t'o'higher temperaturemto regenerate the dentlose. 'Ifeinperatur'esof 100-"1'50" maybe used ior'ihis step but the range of130-150 C. is preferredfl Higher temperatures are less desirable because of their greaterdegrading action on the cellulose,particularly if air is present." Lower temperatures can be used but considerably longer times are required to obtain the necessary decomposition of the cellulose ganthate.""The (1ecomposition of the xanthate talgespl ace rapidly attemperatures above 100 C.;e. g., only one mih ih hea i u d whe fi m is b iiii ii'io l l of oi Q* Q Q' rh i h qi 100 /m Th s oo r ene at on may be obtained by any" convenient known niainsffor example, by ei'rposing'the dried viscose o a"l1ot"air current "or passing ifidi 'iii smbotli or embossed heated rol'l's This 'stageof the process requires careful adjustment and control of time and temperature of heating to ensure obtaining sufiicient Xanthate regeneration without excessively degrading the cellulose. A

simple test for use in determining whether the cellulose regeneration has been carried far enough is to determine whether a small piece of the film has been made insoluble in water. The amount of degradation may be determined by measuring the cuprammonium viscosity of the regenerated cellulose and calculating the degree of polymerization of the cellulose from these viscosity data. The regenerated cellulose should have a degree of polymerization of at least 300 glucose units.

The dry regenerated cellulose structures are washed and purified by conventional methods. They may be washed free of salts with water, and preferably with dilute acid baths to decomhos oi hsio i e hs i ibls s i h poundsattaeli d to the jeellulose. i ammonium sulfate bath maybe used prior to the dilute acid loath to remove salts likely to produce gaseous products in the acid bath which might produce bubbles in'"tl'ie"filin. If a desulfured product is seared, the purified cellulose structure may be tio al desulfuring baths such Qtdilute caustic, by ordinary e urified and washed films are then dried preferably under slight by" two dimensional tension, for example, by passir'ig'them over heated polished rolls.

In order to more fully explain the nature and illustrate the prac'u qr ph ow ng iihmh e a .e v'o oihhosi ono or? Pi? is? .1

indicatedf" Wheel A 1. 995? So ion con ain n 1% of oo ion ih h f ohl o o iiisi ii eq in the a kali el u ose t ge a desire o Polyme ization of .5. glucose nit and ii% of odium hydroxide a d inehosi o a a t iholois o -0 i soo on a las plate and ide ii 'siiiohm Q ir at .5? circulatin t bout 09 t-zmih .17 12 minu e Under those orihqi iohs tho W il??? content of t e, film i r lined .6. th n. .5% befo e ir eversibl soihti h ihkos p eas a d re enerati n 1- o. o: .wmho iiioii of th Xhmhh e s th ro i o eiiosiioit a Endorse hr he re en ion r a ou h. coh nt shoot u n imm rs on in wa e After f iiii ii io Wash in water to' orno o soluble So i nd lkali and to r mo e he shee from he u p rt. it i i en a 3-minutetreiitmoh in mid ath oonta ih 13% sul- Iiir o ooisi and .8% sodium u ia e to dec mpos residual and insoluble sulcfur compounds. The sh ot s. then uhio od to tw f ther. water was e t remove the mid QQ HQ I ZQ y treat: ment a 9 aq eous sod um h droxid soluiioh. at o minute a d fina ly iven two mor wa r The i m is. then ried t 7-9? 9- oh. a und r sli t ten i n. The. tearin trength de erm ned hsosoribed in the followin para raph) of th res lting fi m is '17 rams- "Or ih r ni -so t ne wet-c st cellulose film a t e. som thickness (0.000.88") has a t arin n th o bou 23 ams. The film is h iiooabl to gh a d elastic even When et.

he. tearin stren th of tho re en rated colliilos fiihi is. d t i ed 0P he n rumen which is ih i ii e i i 9 he Ehh n or a r s or This'tesr tester is -afor'ce integrating instrument and the tearing strength as determined with it is the force in grams integrated over the distance an initial tear is extended and is reported as the total force in grams required to extend the tear. The instrument is calibrated to give a direct reading of the tearing strength in grams for a 2-inch tear when a standard 0.0'00 88 inch thick test specimen is used. This specimen average strength reported. Films of other thicknesses than the standard 0.00088 inch may be tested by proper adjustment of the tester. The tearing strengths of films of different thicknesses are directly proportionl to the squares of their thicknesses.

Example II A viscose solution of high salt index (6.6) of the same cellulose and alkali composition as that in Example I and containing 0.2% (based on the weight of the cellulose) of a sulfonated oil wetting agent, is cast on a smooth support and dried for minutes in a stream of air at 110 C., circulating at a rate of 1000 ft./min. These heating conditions are sufficient to dehydrate the viscose to a moisture content of about 20% before injurious xanthate decomposition takes place. Substantially complete regeneration then takes place during the latter stage of the 10-minute heating period. The dehydrated, hotair regenerated cellulose sheet is immersed in 18% ammonium sulfate solution for 3 minutes to remove gas-forming products and. then immersed for 3 minutes in a bath containing 4% sulfuric acid and 6% sodium sulfate to decompose residual insoluble sulfur compounds. After further purification as in the preceding example, the film is immersed for '5 minutes in a 6% aqueous glycerol solution to soften it and is then dried at 85 C. in contact with a smooth surface. This film has a tearing strength under ordinary conditions of temperature and humidity of 42 grams, whereas that of softened wet-cast cellulose sheet of the same thickness has a tearing strength of only about 3 grams. Furthermore, X-ray patterns obtained from this film show practically random orientation of the cellulose crystallites in the sheet. The image obtained from the dryca'st film is fuzzier than that obtained from a conventional Wet-cast film which indicates less ordered arrangement in the crystallites in the dry-cast film than in ordinary wet-cast films.

Example III This example illustrates the preferred process in which the drying stage is separated from the heat-regeneration stage.

A viscose solution having a salt index of 7.6 and otherwise of the same composition as the viscose solution described in Example II is cast on a smooth surface and subjected to a stream of low-humidity air at room temperature moving at a rate of 1000 ft./min. for a period of 30 minutes. At the end of this time the film is tack-free and contains 3540% cellulose or 2010% water. The dry cellulose xanthate film is stripped from the casting support and regenerated by subjecting it to a stream of hot air at 100-1 05 C., circulating at a rate of 1000 ft./min., for one minute. The resulting regenerated cellulose film is subjected to the same Washing and purifying treatment described in the preceding example. The unsoftened film is dried in contact with a smooth surface at 105 C. The

tearing strength of the resulting film is equal in both, directions and approximately 10 times rectionY.

6 that of wet-cast cellulose film of the same thickness. (which shows different properties in the two directions, machine direction and transverse di- Example IV This example illustrates the preparation of cellulose films by a drum-casting procedure and is to be read with reference to the accompanying drawing wherein is diagrammatically illustrated an arrangement of film-forming apparatus particularly suited for the practice of my invention.

An unaged viscose solution containing 8.5% cellulose (having a degree of polymerization of about 500-550 glucose units) and 6.5% sodium hydroxide and having a salt index of 5.0 is prepared from a commercial wood pulp (Brown hardwood pulp) by customary metho s. This solution is cast at room temperature from a hopper I onto the polished surface of a heated stainless steel drum 2 (90 C.) rotating at a surface speed of 2.6 ft./min. The drum is enclosed by a jacket 3 which permits the circulation of heated air over the surface of the film. The film is rapidlydried by a current of air at about 110 C. moving over its surface at a speed of approximately 1500 ft./min. As soon as the water content of the film reaches approximately 30%, the sheet is stripped from the drum and the cellulose regenerated by passing the dried film over rolls 4, 5 and 6 heated to'150 C. The washing and purification of this sheet is carried out in the same manner as in the preceding example (i. e., by passing the film successively through an aqueous ammonium sulfate bath 1, an aqueous sulfuric acid-sodium sulfate bath 8, waterwashes 9 and I0, desulfuring bath ll, water wash l2, and softener bath [3) with the exception that the aqueous 18% ammonium sulfate solution isheld at 90 C. The unsoftened film is dried in contact with polished roll [4 at a surface temperature of 100 C. The. resulting fi1m,,

0.0012" thick; possesses exceptionally good tearing strength and tumbling durability values even though no attempt was made to avoid considerhave durability values of 372 at F., 35% e R. H., 122 at 0 F., 35% R. H., and '72 at F., 7% R. H. Corresponding values obtained with unsoftened wet-cast cellulose film of 0.00088" thickness are: 20-30 at 75 F., 35% R. H., 1 at 0 F., 35% R. H., and 1 at 85 F., 7% R. H. These values would not be appreciably higher at a film thickness of 0.0012".

The regenerated cellulose films produced by the process of this invention are characterized by high tearing resistance and toughness under extreme conditions of temperature and humidity, even when they are not softened. However, for

- some purposes itmay be desirable to incorporate a softener in the film. In these cases the film maybe softened with conventional cellulose softeners such as glycerol, in the customary manner.

The superior properties of films produced by this process in its preferred embodiments in comparison with representative wet-cast cellulose filmsare shown in the following table. Both types of film described are of the standard thickness of 0.00088".,. f

Comparison of properties of dry-cast cellulosefil'm with those of ordinary wet-cast films Films Dry-cast-by Process of Wet cast Fnms This Invention Property f Softened with smelled with Unsoftened myceml Unsoftened 15% Glycerol MD L 1 T MD TD MD I TD MD TD Tear Resistance at 75 F., R. H; in 1 5 grams 1.5 2.0 2 4 3s a4 31 31 Tumbling Durability atr'.,. 5% R. H-. 20-30 70 250- 0 111,357; 12.11... 1 5-15 250 sse-r.,1% R. n.-. 1 3-15 250" 0 e., 1% 1 1 100 Percent Swelling in Water 1 5 I 5 2o 2o Anchorage o'f Coating Fair Fair Good Good. X-Ray. Orientation Planar, with Planar, with -Ra'ndom Random.

some unisome LlIllaxial. axial.

sions in the subsequent stages of wet processing and. drying without substantially losing its desirable properties, i. e., it retains a high, though less uniform, tear resistance and good toughness under extreme atmospheric conditions.

While the invention has been described particularly from the standpoint of production of films or. sheets it is equally applicable to the preparation of filaments, fibers or other shaped -.articles.

Inview of the high durability and tearing rstrength, even in very thin sheets, for example :0'L0005"jthick,. and without softener, the dry-cast cellulose sheets obtainable by the present invenltlbll are particularly usefulv for heavy duty Wraps of all kinds, such as, for example, for wrapping Iheavy machine parts having sharp corners and :for dried vegetable packages and textile wrapswhich normally become embrit-tled by softener "transfer from the film to the fabric or contents. LIti's "also useful for inexpensive shower curtains :and single use shower capes. Because of the better anchorage of 'moistureproo'f coating to :the dry-cast film in comparison with ordinary softened cellulose film, the film of this invention .is used where good waterproofnessis needed, :such as, for example, in tablecloths, aprons or floor coverings. These films are also useful in applications where good sewing properties such .as seam strength are required. Since the drycast film's also have greater elasticity when wet than ordinary cellulose films, they are particularlyuseful for packin meat, especially" as 'casings for sausage, etc. Other uses include their lamination with other materials in thin sheets for extra heavy duty or for, special applications requifingirnpervious'ness to gases; I

Since it is obvious that various changes and weight before the film has lost its solubility in' water, said drying being accomplished by subjecting the film to the action of a stream of inert gas moving ata speed of at least 400 ft./min. relative to the film, saidfinert gas being at a temperature between ordinary room temperature and C., and regenerating the cellulose by heating the driedffilm in an inert atmosphere until regeneration is substantially complete, said extruding, drying, and regeneration being accomplished under substantially tensionless conditions whereby to form a tough, tear-resistant regenerated cellulose film having substantially uniform strength in all directions and further characterized by having the micelles in random distribution.

2'. A process for preparing films of regenerated cellulose from viscose which comprises extruding. viscose on a support to form a viscose film, drying the 'filmto a water content of less than 45% by weight before the film has lost. its solubilityin water, said drying being accomplished by subjecting the film to the action of a stream of inert gas moving at a speed of at least 400 ft./rn in. relative to the film, said inert gas being at a temperature between. ordinary room temcellulose from viscose which comprises extruding viscose on a support to form a viscose film, drying the viscose film to a Water content of less than 20% by weight before the film has lost its solubility in water, said drying being acco1nplished by subjecting the film to the action of a stream of inert gas moving at a speed of at least 1000 ft./min. relative to the film, said inert gas being at a temperature between ordinary room temperature and 130 C., and regenerating the cellulose by heating the dried film in an inert atmosphere at a temperature of 100-150 C., until regeneration is substantially complete, said extruding, drying, and regeneration being accomplished under substantially tensionless conditions whereby to form a tough, tear-resistant regenerated cellulose film having substantially uniform strength in all directions and further characterized by having the miscelles in random distribution.

4. A process for preparing films of regenerated cellulose from viscose which comprises extrud ing viscose having a salt index above 1.0 and wherein the cellulose has a degree of polymerization of at least 500 glucose units, on a support to form a viscose film, drying the film to a water content of less than 45% by Weight before the film has lost its solubility in water, said drying being accomplished by subjecting the film to the action of a stream of inert gas moving at a speed of at least 400 ft./min. relative to the film, said inert gas being at a temperature between ordinary room temperature and 130 C., and regenerating the cellulose by heating the dried film in an inert atmosphere at a temperature of 100-150" C., until regeneration is substantially complete, said extruding, drying, and regeneration being accomplished under substantially tensionless conditions whereby to form a tough, tear-resistant regenerated cellulose film having substantially uniform strength in all directions and further characterized. by having the miscelles in rand-om distribution.

5. The process of claim 4 wherein the viscose has a salt index above 2.0 and the cellulose thereof has a degree of polymerization Within the range of 550-600 glucose units.

5. A process for preparing films of regenerated cellulose from viscose which comprises extruding viscose having a salt index above 1.0 and wherein the cellulose has a degree of polymerization of at least 500 glucose units, on a support to form a viscose film, drying the viscose film to a Water content of less than 20% by weight before the film has lost its solubility in water, said drying being accomplished by subjecting the film to the action of a stream of air moving at a speed of at least 1000 ft./min., relative to the film, said air being at a temperature between ordinary room temperature and 130 C., and regenerating the cellulose by heating the dried film in air at a temperature of -150" C., until regeneration is substantially complete, said extruding, drying, and regeneration being accomplished under sub stantially tensionless conditions whereby to form a tough, tear-resistant regenerated cellulose film having substantially uniform strength in all directions and further characterized by having the miscelles in random distribution.

JAMES BURTON NICHOLS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,576,531 Mendel Mar. 16, 1926 1,864,244 Henderson June 21, 1932 1,937,836 Morse Dec. 5, 1933 2,144,356 Alles Jan. 17, 1939 2,284,028 Ubbelohde May 26, 1942 FOREIGN PATENTS Number Country Date 394,714 Great Britain July 3, 1933 412,798 Great Britain July 5, 1934 728,682 France Apr. 18, 1932 

